Plant and human gene modification goes past transgenics

Agricultural biotech companies have been trying to get around the GMO controversy with newer technology that modify genes in place, without transferring genes from other species. But even without the protests over the putative dangers of GMOs -- some ungrounded in science -- human gene therapy is moving in the same direction.

A new study in the journal Molecular Therapy reports that a genetic defect in cells of Duchenne muscular dystrophy patients has been repaired by genome editing with transcription activator-like effector nucleases, archly called TALENs.

"We show that genome editing with transcription activator-like effector nucleases (TALENs), without a repair template, can efficiently correct the reading frame and restore the expression of a functional dystrophin protein that is mutated in DMD," stated Duke University researchers led by Charles A. Gersbach in the study, the full text of which is open access.

These enzymes actually correct the genetic defect at a targeted location on the gene. Repaired cells make the normal kind of dystrophin, a protein important to muscle function. While this experiment was done in cells, not in vivo, it could actually lead to a cure of this genetic disease.

Another kind of genetic treatment already in the clinic shows signs of providing relief. This approach uses antisense oligonucleotides to selectively block synthesis of portions of the dystrophin protein. This "exon-skipping" approach takes advantage of a peculiar fact: mistakes in building certain segments of the dystrophin are more damaging than deleting them altogether. Sarepta Therapeutics is testing one exon-skipping drug, eteplirsen.

Sarepta's progress is being watched by Isis Pharmaceuticals, the granddaddy of antisense, because Isis has the right to collect milestone payments up to an aggregate of $23.4 million, along with royalty payments on each drug that work through RNA splicing, which includes eteplirsen.

But if the technology reported in Molecular Therapy can be safely and effectively applied to people, drugs such as eteplirsen, which must be taken continuously, could be superseded.

Of course, it will likely take years to get any such drug ready for human testing, and several more years will be needed to see if the drug is safe and effective. (And if eteplirsen and other exon-skipping drugs work, the safety bar will be set higher, because an effective treatment changes the risk-benefit equation).

And this brings me back to the ag-biotech companies trying a similar approach. These companies, such as San Diego-based Cibus and spinoff Nucelis, say they can change genes at a specific location by inducing a DNA mismatch. This stimulates the cell's genetic repair mechanism to make errors in repairing the mismatch. Some of these induced errors will provide an improved function, such as herbicide resistance.

At Nucelis' grand opening last week, CEO Sean O'Connor hinted that the company could take its technology into human therapeutics.

"In theory, even though we've not moved in that direction yet but it's on our radar screen, we could apply it to animal and human cells," O' Connor said.

Now here's a more technical description of the similarities of the Cibus/Nucelis technology with its application to human genetic diseases. The Cibus approach uses single-stranded oligonucleotides to induce the mismatch. A 2003 article in Gene Therapy describes how such oligonucleotides corrected the mutation that causes Pompe disease, in fibroblasts taken from a patient.

"The studies demonstrate the utility of single-stranded ODN (oligonucleotides) to direct targeted gene correction or mutation in a human hereditary disease and in an animal model. Our data open the possibility of developing ODN vector as a therapeutic approach for treatment of human hereditary diseases caused by point mutation," the paper stated.

Combining the oligonucleotide and zinc finger approaches allows efficient genetic modification of human cell lines, according to a July 17, 2011 article in Nature Methods, available at PubMed.

After reading about zinc fingers, TALENs and single-stranded oligonucleotides, keeping a clear picture of what the technologies do and what distinguishes them. This presentation on plant biotech provides a good walk-through of what they are.